There have been many attempts in the art to provide polyorganosiloxane-based graft polymers which may be useful as impact strength modifiers for thermoplastic resins. See, for example, U.S. Pat. No. 2,891,920 (J. F. Hyde, et al.); and O. Graiver, et al., Rubber Chem. Tech., 56 (5), 918 (1983).
U.S. Pat. No. 3,898,300 states that a polyorganosiloxane-based graft copolymer for improving the impact strength of S/AN resin is formed by grafting S/AN comonomers in an emulsion system onto the vinylsiloxane or allylsiloxane containing silicone substrate. U.S. Pat. No. 4,071,577 describes a similar approach by using a mercaptosiloxane in place of vinyl-group containing siloxanes. European Patent 0,166,900 reports further improvement of polysiloxane-based graft polymers and increased S/AN impact strength by using acryloxyfunctionalized siloxane as the graft-linking agent. These graft polymers are utilized in connection with the impact modification of S/AN. British Patent No. 1,590,549 describes the use of a polyorganosiloxane graft copolymer in various plastic molding compositions. Similarly, European Patent Application 0,249,964 describes the use of a polyorganosiloxane graft copolymer in the polycarbonate containing blends.
None of the references disclose the in-situ co-homopolymerization of vinyl monomers in the presence of siloxanes in an emulsion system, as described hereinbelow. The present invention is also directed to the use of graft polymers provided by subsequent graft polymerization of vinyl monomers (e.g. polymethyl (meth)acrylate, polystyrene or styrene/acrylonitrile copolymer) in the presence of such a co-homopolymerized polyorganosiloxane/vinyl-based substrate.
The use of a diene or acrylic rubber-based impact modifiers in thermoplastic resins has become a common practice in the art. The selection of either material depends largely on the end use purposes, such as weatherability or low temperature impact resistance. Uniform color appearance of molded parts is a benefit which is gained by using a diene-based impact modifier over acrylics. However, the unsaturated moieties of the diene rubber restrict its outdoor use to some extent due to its tendency to oxidize and yellow.
Surprisingly, it has been found that partial replacement of the oxidation or ozone sensitive rubber by a silicone-based rubber effects more improvements, such as low temperature ductility, low gloss, and impact and discoloring resistance against thermal aging on the blends described hereinafter. Unexpectedly, it is now possible to prepare low gloss polycarbonate and polyester blends having both excellent low temperature impact resistance and discoloration resistance by the addition of an effective amount of a silicone-based impact modifier to a diene-based impact modifier. Mention is also made to EPO 0,260,558 which discloses a combination of a silicone-based impact modifier with an alkylacrylate-based modifier. The patentee, however, makes no mention of the use of a diene-based impact modifier.